Recently Google advised websites that it would take HTTPS into
consideration when displaying its search engine results page.
The direct indication from Google can be summarized as follows:

If an SSL version of a web page exists, Google will show that
page in the index.

That was it, a very simple concept. But we wondered, what
kind of impact would an HTTPS version have on the actual ranking
within a SERP? So we did some testing to find out. From
our early returns, we have seen some indications that HTTPS does
have a favorable impact on SERP's. But due to the constant
changes on SERP's, it is hard to identify exactly the impact.
And as is the case with other ranking factors, its almost
impossible to quantify.

Our conclusion from this study is as follows: get HTTPS for your
website! It may not have a major impact on your search
rankings. But Google recommends it. And whatever Google
recommends, we recommend. After all, you are creating a more
secure experience for your users.